Attention has been paid to carbon nanotubes as a new material having many excellent characteristics originating from their specific structure.
As a process for growing carbon nanotubes, known are an arc discharge method, a laser ablation method, a liquid phase method and others. From the viewpoint of productivity, controllability and consistency with semiconductor processes, chemical vapor deposition (CVD) is superior. Out of CVD methods, a catalytic CVD method is general, wherein catalytic fine particles made of Fe, Co, Ni or the like are deposited on a substrate, and carbon nanotubes are grown thereon. At present, high-quality carbon nanotubes can be relatively easily obtained, which have a density of the first half of the order of 1011 cm−2.
About the catalytic fine particles, known are a method of depositing fine particles directly onto a substrate by sputtering, an arc plasma gun or the like, and a method of treating a catalytic thin film with plasma to produce fine particles. In order to grow carbon nanotubes so as to have a high density of 1012 cm−2 or more, it is necessary to set the diameter of the catalytic fine particles into a nano-order size. However, it is known that the nano-order size catalytic fine particles are easily aggregated by their high surface energy. The aggregation tends to be made remarkable by external factors in the step of growing the carbon nanotubes, in particular, a raised temperature. Therefore, the growth of carbon nanotubes into a high density of 1012 cm−2 or more is very difficult.
Thus, as a method for restraining the catalyst aggregation, suggested are a method of carrying catalytic fine particles onto a porous material (for example, zeolite) in Chem. Phys. Lett. 381 (2003) 422, a method of adopting a sandwich structure wherein a catalytic layer is sandwiched between Al2O3 layers in Jpn. J. Appl. Phys. 44 (2005) 1558, and other methods. However, even according to these methods, it is difficult to prepare a catalyst having a density of 1012 cm−2 or more. When it is supposed that carbon nanotubes are applied or developed to various articles or purposes, it is preferred that such a carrying material or lamination structure is not used.